1. Field of the Invention
This invention relates to a hydropyrolysis process and, more particularly, to a hydropyrolysis process under carefully selected and controlled conditions of temperature and pressure wherein heavy, high molecular weight feedstocks are cracked in the presence of hydrogen to yield lighter, lower molecular weight, liquid products.
2. The Prior Art
Thermal cracking was the primary process for production of gasoline from crude petroleum until the late 1930's. Thermal cracking was employed to increase the yield of gasoline either by direct processing of heavy feeds, or indirectly, through the production of light olefins, which were then subjected to polymerization. Subsequently, it was gradually replaced by the more efficient catalytic cracking and reforming. Thermal processes of importance during and before the Second World War included cracking, visbreaking, coking, reforming, alkylation and polymerization. Thermal reforming processes were used to convert low quality gasoline and naphtha into high-octane gasoline by various transformations, e.g. isomerization and dehydrogenation, while thermal alkylation was employed in the production of blending components for aviation fuel. Another important thermal process, used in England during the Second World War for the manufacture of aromatics and olefins, was the Catarole process. In this process, highly naphthenic feeds were cracked to mono- and diolefins, which, through resynthesis at extended reaction time, gave monocyclic and polycyclic aromatic compounds.
At present, thermal cracking processes represent a relatively minor part (less than 10%) of the modern refining capacity in the United States. Such processes are being used for upgrading the heavy liquids and for production of petrochemicals. In particular, visbreaking and coking are two important applications for the production of fuels from heavy oils. Visbreaking is a mild form of thermal cracking which reduces the viscosity of feedstocks, such as vacuum resids and heavy gas oils. The process yields mainly middle distillate fuel, accompanied by lower amounts of gasoline, making it a suitable process in case the gasoline demand is low compared to that for middle distillate. Coking processes are based on the principle of carbon rejection, i.e. increase in the hydrogen/carbon ratio of distillable liquid products at the expense of partial carbonization of the starting material. Coking is applied for upgrading of feeds such as reduced crudes, vacuum resids, shale oils, tar-sand liquids, coal tar, and gilsonite. When such heavy liquids are heated to 480.degree.-565.degree. C. there is extensive cracking of large molecules yielding free radicals, which are stabilized by abstraction of hydrogen from other molecules. Continuation of this hydrogen transfer process leads to a liquid product (gas oil) which is richer in hydrogen, and a solid product (coke) which is poorer in hydrogen, as compared to the feed.
Another important thermal process is the steam cracking of C.sub.2 -C.sub.4 paraffins, naphtha, and gas oil for the manufacture of C.sub.2 -C.sub.4 olefins, which are important starting materials in the petrochemical industry.
With decreasing petroleum resources, increased interest is being directed toward the production of synthetic crudes from coal, tar sands, and oil shale. These crudes, because of their high viscosity and high molecular weight, present unique production, handling and processing problems. Present processes for upgrading heavy crude liquids are based either on addition of hydrogen or rejection of carbon. The addition of hydrogen to these heavy materials has proven to be very expensive. Accordingly, carbon rejection (coking) is currently the most popular method for upgrading heavy crudes. The disadvantage of coking is that it converts a substantial portion (10-25%) of the feed material to coke.
In view of the foregoing, it would be a significant advancement in the art to provide a process which will either totally or at least partially eliminate coke formation while increasing the liquid yield from high molecular weight feedstocks. It would also be an advancement in the art to provide a non-catalytic process for producing lower molecular weight, liquid hydrocarbons from higher molecular weight hydrocarbons in the presence of heavy metal contaminants. Such a process is disclosed and claimed herein.